Sweep circuit



Nov. 8, 1960 Filed E. G. I URcc'r-r, JR., ETAL 2,959,750

SWEEP CIRCUIT Jan. 17, 1957 2 Sheets-Sheet 1 4 v #Fr-#P Nov. 8, 1960 E. G. LURcoTT, JR., ETAL 2,959,750

swEEP CIRCUIT MPL/riff? 4J 1| IN VEN T0125 SWEEP CIRCUIT Woodbury, NJ., and Stanley Dobren, Philadelphia, Pa., assignors to Radio ration of America, a corporation of Delaware Filed Jan. 17, 1957, Ser. No. 634,795

9 Claims. (Cl. S32- 42) The present invention relates to a circuit for converting three carrier wave signals modulated by voltages in threephase relation to two carrier wave signals amplitude modulated by phase displaced voltages. The invention is especially useful for producing amplitude modulated sweep voltages.

In a radial time base radar display, as, for example, a plan position indication (PPI) display, a sawtooth wave deects the cathode ray beam radially outward from its center position in an angular direction which corresponds to the antenna beam direction. One method for obtaining this type of deilection is to employ magnetic beam deection coils which are rotatable about the neck of the display tube in synchronism with the antenna rotation. The synchronism between the deflection coils and the radar antenna is ordinarily maintained by means of a servo system. whether magnetic or electrostatic in nature, are held tixed and the sweep signal is modified to produce the radial, rotating trace. In these systems, the sawtooth wave may be applied to a two phase resolver, the rotor of which rotates with the radar antenna. The horizontal and vertical deflection outputs from the resolver are sawtooth signals whose envelopes are the electrical analog of the antenna motion. It is sometimes possible to attach the resolver directly to the antenna shaft. This makes the system relatively simple, as no servo system is required. However, when, as is the usual case, the indicator must be at a relatively large distance from the radar antenna, excessive signal attenuation and loss of bandwidth become serious problems. These losses must be compens-ated for and this requires expensive and relatively complicated equipments.

It is an object of the present invention to provide an improved and greatly simplified circuit for producing resolved sweep signals for a radial time base indicator which overcomes many of the problems outlined above.

A more general object of this invention is to provide an improved system for converting three carrier wave signals modulated by voltages in three-phase relation to two carrier wave signals modulated by voltages in any desired phase relationship-90 in one form of the invention.

The invention, in its broader aspects, includes a means, such as a synchro generator driven by the radar antenna, for Iproducing three voltages which are modulated by voltages in three-phase relation. These voltages are applied to a mixing matrix such as an impedance network. In a specific form of the invention, the matrix consists of a three terminal, Y-shaped resistor network, the arms of which are of equal resistance. The carrier wave is also applied to the mixing matrix in such fashion that arent C ice relation. When used in a PPI radar display system, the two output voltages are detected and the respective en velopesA employed to produce the cathode ray beam deecting sweep signals.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing in which:

Figure l is a block and schematic and circuit diagram of a preferred form of this invention;

Figure 2 is a block and circuit diagram which is useful in explaining how the system of the invention operates. Figure 3 is a schematic circuit diagram of a portion of a modified form of the invention shown in Figure l; and

Figure 4 is a schematic circuit diagram to illustrate a general aspect of the present invention.

Similar reference characters are applied to similar elements throughout the drawings.

Referring to Figure l, radar system 10 includes a rotatable directive antenna assembly 12 which is driven by the antenna drive means 14. The latter may be an electric motor or the like. The mechanical connection between the drive means and the antenna is indicated by dashed line 16. The drive means also drives the rotor 18 of the three-phase synchro generator 20. A carrier wave signal ec is applied to the rotor 18 from alternating current source 22. The frequency of source 22 may be the power y line frequency, say 50, 60 or 400 cycles, for example.

A In other systems, the deflection means,

two output voltages are derived from the network, the

produce output modulating signals in phase quadrature "3 The output signal of the synchro generator available at lines 24, 26 and 28 is in the usual form. Thus, each of the three signals is a suppressed carrier wave signal and each is modulated at the frequency at which the antenna drive means 14 drive the antenna. One of the three signals is shown at 30. The modulating voltages of the three signals Iare in three-phase relation..

The three output signals are applied to the three terminals 32, 34 and 360i a Y impedance network 38, the center 40 of which oats with respect to ground. In a preferred vform of the invention, the three arms of the Y comprise resistors 42, 44 and 46, respectively, of equal value. The network 38 is termined a mixing matrix in the discussion which follows. For reasons which will be given in more detail later, the carrier wave signal ec, in addition to being 4applied to the synchro generator, is also applied, via ampliiier 48, to the mixing matrix.

The operation of the above-described system can be more readily understood from the mathematical analysis which follows. The equations should be considered in connection with Figure 2, which is a portion of Figurel 1 to which various identifying reference letters have been applied'. In the analysis, it is assumed that the input impedance to the detectors Sti (Figure l) following the mixing matrix is high, compared to the impedance of the matrix. This is actually the case, and it permits one to neglect the loading effect on the matrix.

Synchro generator 20 may be either Y or delta connected. In the analysis, the generator is considered Y connected, as shown in Figure 2. However, the conclusions reached are equally applicable to the delta connected generator as both networks are equivalent.

The voltages across windings OA, OB and OC of the synchro generator are e0A=A cos (wsI-l-5/61r) cos wct (l) e0B=A cos (wst+1r/6) cos wet (2) e0C=A cos (ast-M2) coswct (3) where A=the maximum amplitude ws=the angular modulating frequency wc=the angular carrier frequency Resistors 42, 44 and 46 are of equal value. k,In other words, the load connected to the generator is balanced.

Therefore the voltages eofA', e013, eofc' across the resistors are inthe same form as 1, 2 and 3 above. Therefore:

The voltages appearing at A' and B' measured relative to node N, are:

eNA'=eNo'+eo'A' (7) 2 l v eNAI=AV +(K+ 1/2)2 4 The value of K can be determined as follows. Equations 14 and l5 by Multiply this gives:

from which we see:

NB1-A -l-(K-l-1/2) [cos 0 cos ast-sm 9 sin wat] cos met where K is the fraction of the total R tapped from O to form node N. Substituting Equation 9 in Equations 7 and 8 gives:

Applying the formula for the cosine of the sum of two angles gives:

where HKM/2)a Using a trigonometric identity for the terms in brackets of Equations 16 and 17 gives:

0 sin1 (18a) For a quadrature relationship between the two modulating signals, it is apparent that since K must be positive,

yvoltages to any desired value.

Placing (21) and (22) in (19') and (2o) gives triennal form of the voltages: l

and the envelopes are in quadrature. The voltages at A' and B relative to ground are;

eGB'=eGN+eNB' (26) where eGN==B cos wet (27) and represents the reinserted carrier signal.

Placing (23), (24) and (27) in Expressions 25 and 26 gives:

t eGB,=B cos (mt) -l-N/rcos (w.tl1/41r)] cos wt collecting terms results en.: Blix/2% @da (w+ 3/41r) +1] eos ,t

eGB': cos (ost-l- 1/41r) -I- l] eos wt which are recognizable as the standard equations for amplitude modulation. The term waa is the degree of amplitude modulation and should be less than unity.

If we apply the formula for the product of cosinesto Expressions 28 and 29, we obtain:

The above are equations for amplitude modulated signals with the modulating voltages in quadrature. Note 'the presence of the upper and lower sidebands.

The above equations demonstrate that when From Equations 19 and 20k above, it is seen that the angle qs between the two modulating voltages is Therefore, the angle qb between the two modulating voltages 1s t/ ai The derivation above can readily be checked by substituting the value of for K in Equation 35. Solving for p we obtain or When K=0, p turns out to be 120, as one would expect, and when K==1, =30. Other values of K give other values of qb.

In the circuit of Figure 1, K is adjusted to havev a The modulating voltages of the carrier wave applied to peak detectors 50 are therefore in quadrature. The peak detectors detect the modulation envelopes and provide two modulation components in quadrature, one of which is shown at 54.

The sawtooth wave to be modulated is generated in a conventional sweep circuit 56. It includes a rectangular wave generator 58, the output of which is applied through keyed clamps 60 and 62 to charging capacitors 64 and 66. Thesel charging capacitors charge toward the instantaneous value of each detector output. The widths of the rectangular pulses generated by rectangular wave generator S8 correspond to the radar range interval of interest.k The generator output is synchronous with the radar pulse repetition rate, the synchronizing pulses being applied via lead 68. The resultant sinusoidally modulated sawtooth signals available at terminals 70 and 72 are in quadrature, one of the signals being shown at 74. Terminal 70 may be coupled to the horizontal dellection means of a cathode ray indicator and terminal 72 to the vertical deiection means of the indicator.

The phase control of amplifier 48, shown in Figure 1, is necessary to compensate for any phase shift which may occur in the synchro generator. The phase control setting should be such that the phase of the carrier inserted at the matrix is the same as the phase of the suppressed carrier signals across NA (Figure 2) and NB (Figure 2). If the phases are not identical, the output voltages from the matrix are distorted. The accuracy with which the phase of the inserted carrier must be fixed relative to the suppressed carrier is determined by the amount of envelope distortion that can be tolerated in a particular application.

The amplitude of the injected carrier should be equal to or greater than the amplitude of the suppressed carrier signals across N'A and N'B' for optimum performance. The greater the amplitude of the inserted carrier relative to the suppressed carrier, the less the distortion of the output of the matrix. This can readily be demonstrated mathematically, however, the analysis is rather lengthy and involved and will not be givenl here.

Capacitor Cc and resistor Rc comprise a coupling network. The resistance of Rc should be sufficiently large to prevent loading of the amplifier 48 andcapacitor Cc should be large enough to couple the carrier signal to Rc without attenuation.

Further details of some ofthe circuits which may be used in the arrangement such as shown in Figure l, are given in Figure 3. Amplifier 48 has an anode load circuit 90 and a cathode load circuit92. The carrier wave is applied to the grid-to-cathode circuit of the ampliiier at terminals 94. The amplitude of the output wave may readily be adjusted by adjusting potentiometer 96. The phase shift producing network including capacitor 98 and resistor 100 is located in the amplifier output circuit. The phase of the carrier wave is adjustable by means of a tap on resistor 100. The square wave generator 99 operates in push-pull rather than being single ended as shown in the embodiment of Figure 1. The clamps 101, rather than being returned to ground, as shown in Figure l, are connected to receive the cathode ray tube centering voltages.

In operation, during the sweep time the clamps are cut olf and capacitors 102, 104, 106 and 108` charge toward the instantaneous peak voltagesat the outputs of detectors 110, 112, 114 and 116, respectively. However, before they charge to more than asmall fraction of the charge they are capable of assuming, the keyed clampers are turned on and they clamp the capacitors to'the centering voltages, discharging' them for the retrace 'or iiyback period. The capacitors are charged to only a` fraction of their peak value in order to maintain their sawtooth output waves linear.

The output voltages at terminals 118 and 120 are applied to the horizontal deflection plates, and the output voltages at terminals 122 and 124 are applied to the vertical deflection plates.

The system above has been described in terms of converting three amplitude modulated suppressed carrier signals to two amplitude modulated signals, the modulation components of which have a predetermined phase relationship. The invention is equally applicable to conventional amplitude modulated signals. If, in the arrangement of Figure 4, conventional amplitude modulated signals are applied to terminals 126, 128 and 130 of the impedance matrix, and point N in the matrix is connected to A C. ground, the output signals to ground available at terminals 126 and 128 will be amplitude modulated signals. the modulation components of which have a predetermined phase relationship. In this case, it is not necessary to reinsert the carrier. When K is equal to the modulation components of the output waves vat .terminals 126 and 128 are in quadrature.

The system shown in Figure 4 is also applicable to the case in which the three input signals are amplitude modulated, suppressed carrier signals. However, if the carrier is not injected at N, the outputs from the matrix are still suppressed carrier signals. It is therefore necessary to use special types of detectors at block 50 in order to recover the modulation information.

What is claimed is:

1. In combination, means producing three amplitude modulated signals whose modulation voltages are in threephase relation; and means including a four terminal impedance network to three terminals of which the three signals are applied and to the fourth terminal of which the carrier component of the signal is applied for converting the three signals to two amplitude modulated signals whose modulating voltages are in quadrature.

2. In combination, means producing three amplitude modulated Sulppressed carrier..frequeay Signalawhgse modulating voltages are in three-phase relation; an im-` pedante network connected to receive said three signals; and means for supplying a signal at the carrier frequency to said impedance network so as to convert said three signals to two amplitude modulated signals whose modulating voltages are displaced in phase from one another.

3. In combination, a circuit for producing three amplitude modulated suppressed carrier wave signals whose modulating voltages are in three-phase relation; a three terminal impedance network to the three terminals of which the respective modulated carrier wave signals are applied; circuit means coupled to a point on said network between two of the terminals thereof for supplying a wave at the carrier frequency to said network; and circuit means coupled to said network for deriving therefrom two amplitude modulated signals whose modulating voltages are displacedin phase from one another.

4. In combination, means producing three amplitude modulated suppressed vcarrier wave signals whose modulating voltages are in three-phase relation; a Y resistor network the three ends of the Y comprising input terminals, and the center of the Y comprising a common connection; means for applying the three amplitude modulated signals to the three terminals, respectively; a tap on one of said resistors; andmeans for introducing the carrier ,wave signal at said tap.

5. In combination, means for producing three amplitude modulated suppressed carrier signals which can be represented by the expressions where A equals a constant, ws equals the angular frequency of thevmodulating signal, t equals time, we equals the angular frequency of the carrier wave, and 0 is any arbitrary phase; means for combining with said three voltages a fourth voltage Eq, where Eq equals F cos wct, and F is at least equal to and means for deriving from the combined voltages two output voltages ER and ES, where ER equals G cos (ust-Hp) and Es equals G cos (wst-l-qS-i-fr/Z), where G is a constant and qb is another arbitrary phase.

6. In combination, means for producing three amplitude modulated suppressed carrier signals which can be represented by the expressions Ex=A cos (mst-I-Ho-l-Z/ 311-) cos wot Ey=A cos (wst-l-Bo) cos wel Ez=A cos (wst-l-@o-Z/Bw) cos wet Where A equals a constant, ws equals the angular frequency of the modulating signal, t equals time, we equals the angular frequency of the carrier wave, and 60 is any arbitrary phase; means for combining with said three voltages a fourth voltage Eq, Where Eq equals F cos wat, and F is at least equal to means for deriving from the combined voltages two output voltages ER and ES, where ER equals G cos (wst-l-q) and Es equals G cos (wst-l-qb-l-ip), where G is a constant, p is another arbitrary phase position, and i/f is an angle.

7. A sweep generator comprising, in combination, means producing three amplitude modulated suppressed carrier frequency signals whose modulating voltages are in three phase relation; an impedance network connected toreceive said three signals; means for supplying a signal at the carrier frequency to said impedancenetwork so as to convert said three signals to two amplitude modulated signals whose modulating voltages are in quadrature; detector means connected to receive said two amplitude modulated signals for detecting the modulation envelopes of said signals; a sawtooth wave generator; and modulating means in circuit with said generator and said detector for producing two amplitude modulated sawtooth wave signals, the modulation components of which comprise sine Waves in phase quadrature relation.

8. In combination, means producing three amplitude modulated suppressed carrier wave signals whose modulated voltages are in three-phase relation; a Y resistor network having resistive arms of equal value, the three ends of the Y comprising input terminals, and the center of the Y comprising a common connection; means for applying the three amplitude modulated signals to the three terminals, respectively; a tap on one of said resistors at a point along its length such that the resistance between said tap and the center of said Y is VEL 1 2 of the total value of said resistor; and means for introducing the carrier wave signal at said tap.

signals available at two of said terminals are amplitude modulated by modulating voltages which are in quadrature; a sawtooth wave generator; and means receptive of said two amplitude modulated signals and in circuit with said sawtooth wave generator for producing amplitude modulated sawtooth wave signals, the modulating voltages of which are in quadrature.

References Cited in the le of this patent UNITED STATES PATENTS Wilson July 22, 1952 Adkins et al Dec. 13, 1955 OTHER REFERENCES Cathode Ray Tube Displays by Soller, Starr and Valley, McGraw-Hill 1948.

UNITED STATES PATENT oEEICE CERTIFICATION OF CORRECTION Patent No., 2,959,750 November 8,I 1960 Eugene G Lureott, Jr..vv et al.

It is hereby certified that error appears in 'the above numbered pateni'J requiring correction and that the said Letters Patent should read as corrected below.

Column 8, between lines 4l to 44, for f"- Y IP read i h2 A,

Signed and sealed this 25th day of April I96l (SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID Lo LADD Commissioner of Patents UNITED STATES PATENT oEEICE CERTIFICATION OF CORRECTION Patent No.. 219594-750 November 8v 1960 Eugene G. Lurcott, JI,r et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that they said Letters Patent should read as corrected below.

Column 8, between lines 41 to 44., for ""1, if Iread Signed and sealed this 25th day of April 1961.

(SEAL) Attest: y

ERNEST W. SWIDER t DAVID L. LADD Attesting Officer Comrnissionerof Patents 

